This disclosure relates to methods for controlling plasma spray coating porosity on an article and articles manufactured therefrom.
Spray coating processes such as air plasma processes, vacuum plasma processes, high velocity oxygen fuel thermal spray processes, and the like, are used to coat turbine buckets. These processes may produce coatings that are partially porous. Porosity in coatings may be detrimental to the performance and life of the turbine bucket. In order to repair turbine buckets that have coatings with excess porosity, it is necessary to strip and recoat them. Stripping and recoating the turbine buckets is time consuming and expensive. Moreover there is no assurance that porosity level will be acceptable after the recoating of the bucket.
The porosity in the coatings is influenced by several factors. One factor is the deposition of non-molten or partially molten particles on the turbine bucket during the coating process. Non-molten or partially molten particles are generally deposited on the coated surface in a porous ring along the edge of the spray cone. Another factor is the rebounding of non-molten particles from concave surfaces on the turbine bucket. Yet another factor is the rebounding of non-molten particles from surfaces upon which the spray impinges. There is also an interplay amongst the aforementioned three factors that may produce porosity in the coating.
It is therefore desirable to develop a method for spray coating turbine buckets that determines the contribution to porosity from each of the aforementioned factors and their respective interactions with one another and that can be used to control porosity in the coating on the turbine buckets.